1. Field of the Invention
The present invention generally relates to a substrate, a semiconductor device, a method of manufacturing a substrate, and a method of manufacturing a semiconductor device, and more particularly to a substrate, a semiconductor device, a method of manufacturing a substrate, and a method of manufacturing a semiconductor device for mounting an optical element for converting optical signals to electric signals.
2. Description of the Related Art
Development of optical communication is advancing along with the increases in speed and size of recent information communication. Ordinarily, in optical communication, electric signals are converted to optical signals, the optical signals are transmitted through an optic fiber, and the received optical signals are converted to electric signals. In the conversion between the electric signals and the optical signals, an optical element having a light emission/reception part is used. The optical element includes, for example, a vertical cavity surface emitting laser (VCSEL), a photodiode (hereinafter referred to as “PD”), and a laser diode (hereinafter referred to as “LD”).
In a semiconductor device provided with such optical element, the optical element is mounted on the semiconductor device in a manner where the light emission/reception part of the optical element faces toward a core part of an optical fiber provided in a through-hole penetrating a substrate of the semiconductor device.
A conventional semiconductor device 10 including optical elements 15 and 16 is described with reference to FIG. 1. FIG. 1 is a cross-sectional view showing the semiconductor device 10 having the optical elements 15, 16. The semiconductor device 10 mainly includes a substrate 11, optical elements 15, 16 having light emission/reception parts 15A, 16A, an optic waveguide 17, and mirrors 21, 22.
The substrate 11 has a multilayer structure having multiple wires and a resin layer provided to a resin substrate. The substrate 11 is formed with through-holes 12a, 12b that penetrate through the substrate 11. The through-hole 12a is provided with an optical fiber 13, and the through-hole 12b is provided with an optical fiber 14. The optical fibers 13, 14 include core parts 13a, 14a, and clad parts 13b, 14b that cover the core parts 13a, 14a. Optical signals are transmitted by the core parts 13a, 14a. 
The optical elements 15 and 16 are mounted on the substrate 11. The optical element 15 is mounted on the substrate 11 in a manner that the light emission/reception part 15A of the optical element 15 faces toward the portion of the core part 13a that is situated at an end part 13A of the optical fiber 13. Furthermore, the optical element 16 is mounted on the substrate 11 in a manner that the light emission/reception part 16A of the optical element 15 faces toward the portion of the core part 14a that is situated at an end part 14A of the optical fiber 14. The mirror 21 is provided at the end part 13B of the optical fiber 13 and the mirror 22 is provided at the end part 14B of the optical fiber 14. The mirrors 21, 22 are provided for enabling optical transmission between the optic waveguide 17 and the optical fibers 13, 14.
The optic waveguide 17 is provided between the mirror 21 and the mirror 22. The optic waveguide 17 includes a core part 18 and a clad part 19 that covers the periphery of the core part 18. The mirrors 21 ,22 are connected to the core part 18 in a state that allows optical signals to be transmitted therethrough (See, for example, Japanese Laid-Open Patent Application No. 2004-54003).
In the semiconductor 10, it is desirable to reduce deviation between the positions of the core parts 13a of the optical fibers 13, 14 (attached to the through-holes 12a, 12b) and the positions of the corresponding light emission/reception parts 15A, 16A facing the core parts 13a, 13b, so that transmission loss between the optical fibers 13, 14 and the light emission/reception parts 15A, 16A can be reduced.
FIG. 2 is a plane view of the substrate 11 having the optical fiber 13 attached to the through-hole 12a. However, in the semiconductor device 10, since the through-holes 12a, 12b corresponding to the optical fibers 13, 14 are formed by irradiating a laser (e.g. a YAG laser, a CO2 laser, an excimer laser) to a resin substrate, it is difficult to form the through-holes 12a, 12b in a precise predetermined position facing the light emission/reception parts 15A, 16A of the optical elements 15, 16 and it is difficult to control the size of the diameters of the through-holes 12a, 12b. 
Furthermore, since a large space L1 (e.g. 10 μm) is provided between the wall of the through-hole 12a, 12b having diameter R2 and the optical fiber 13, 14 having outer diameter R1 for enabling attachment between the optical fibers 13, 14 and the through-holes 12a, 12b, it is difficult to restrict the position of the optical fibers 13, 14 with the walls of the through-holes 12a, 12b. Accordingly, the position of the core parts 13a, 14a of the optical fibers 13, 14 attached to the through-holes 12a, 12b tends to deviate from the position of the corresponding light emission/reception parts 15A, 16A, thereby making it difficult to reduce transmission loss of optical signals.
Furthermore, even if the position between the core parts 13a, 14a and the light emission/reception parts 15A, 16A is optimized, the resin substrate in which the through-holes 12a, 12b are formed may change the position of the optical fibers 13, 14 attached to the through-holes 12a, 12b in a case where thermal contraction or thermal expansion of the resin occurs when the temperature of the substrate 11 changes. This results in an increase of transmission loss of optical signals.